羌塘盆地沃若山地区上三叠统土门格拉组烃源岩沉积环境分析

Sedimentary environments of the Upper Triassic Tumengela Formation in the Woruo Mountain area in the Qiangtang Basin,Xizang

关键词:  羌塘盆地  上三叠统土门格拉组  地球化学特征  沉积环境

Key words:  Qiangtang Basin  Upper Triassic Tumengela Formation  geochemical signature  sedimentary environment

摘要:

岩石学、沉积相、元素地球化学的综合分析结果表明,羌塘盆地沃若山地区上三叠统土门格拉组(T₃t)地层为三角洲沉积;整体富含水解性元素、Fe族元素以及上部砂岩Fe元素富集(高Fe/Mn比值)的特征表明古水体环境为较浅的河口-滨岸环境;CIA值(63.8~79)、ICV值(0.52~1.4)和Ga/Rb-K₂O/Al₂O₃关系图解分析结果显示当时为风化强度中等、较温暖的半干旱-半湿润气候;Sr/Ba比值全部小于1(0.09~0.27),表明沉积水体为盐度较低的陆相淡水;根据V、U、Mo的含量比值,判断其为氧化-弱氧化的富氧水体环境。较为温湿的气候有利于生物的繁殖,但氧化-弱氧化的环境不利于有机质的富集,限制了该地区烃源岩发育:土门格拉组烃源岩的TOC含量为0.76%~1.46%(均值为1.04%),属中等品质的烃源岩。

Abstract:

This paper focuses on the study of the sedimentary environments of the source rocks from the Upper Triassic Tumengela Formation (T₃t) in the Qiangtang Basin, Xizang, in the light of petrology, geochemistry and sedimentary facies analysis. The sedimentary environments of the source rocks in the Upper Triassic Tumengela Formation is assigned to the delta sedimentary facies. The enrichment of the hydrolytic elements, iron group elements, and the element Fe (Fe/Mn ratios) in the upper sandstones indicates the shallower river mouth to onshore environments. The values of CIA (63.8-79), ICV (0.52-1.4) and Ga/Rb vs. K₂O/Al₂O₃ diagrams show the moderate weathering under the semi-arid to semi-humid climatic conditions. The Sr/Ba ratios are all less than 1 (0.09 to 0.27), indicating the low-salinity and fresh continental sedimentary environments. The V, U and Mo contents also indicate the anoxic-suboxic water environments. The warmer and wetter climates are beneficial to the reproduction of organisms, while the anoxic-suboxic environments are not constructive to the enrichment of organic matter. The low TOC values of 0.76% to 1.46% (mean value is 1.04%) have disclosed that the source rocks should be assigned to the medium-quality source rocks in the study area.